There have conventionally been proposed several techniques pertaining to pre-search of narrowing down search targets in advance under conditions with a light processing load before a search process under conditions with a heavy processing load.
For example, Japanese Patent Laid-Open No. 2001-319231 discloses a technique of saving a document within a database in advance together with the number of regions of the document, segmenting an input image into regions to calculate the number of regions, narrowing down search targets to ones having the same number of regions, and then comparing feature amounts. This technique can narrow down search targets by a very simple process.
To search a database which increases in size due to an increase in business documents and the like, narrowdown by the conventional technique is not sufficient, and a higher-precision narrowdown technique is required. The conventional technique does not pay any attention to a region detection error or excessive region detection in a region segmentation process, and search omission may occur.